Field‐Effect transistors as transducers in biosensors for substrates of dehydrogenases

A specially designed field‐effect transistor (FET) with a significantly enlarged gate area was applied in a classical urea enzyme FET (ENFET). The resulting high stability and sensitivity toward pH shifts make it predestinated for the measurement of H + produced in the equilibrium of NAD + ‐dependent enzymatic reactions, especially when the equilibrium is shifted by a subsequent reaction. As a model, the glucose dehydrogenase (GDH) reaction connected to an ion‐sensitive field‐effect transistor (ISFET) is demonstrated by which glucose could be determined in the range from 1 to 40 mM. A platinum electrode on the gate of the FET permits the measurement of reduction equivalents (NADH) by means of the recently, reported chronopotentiometrical methods. Thus, in principle, a way toward a redox ENFET is shown.